Semiconductors, such as silicon, germanium, or gallium arsenide, are utilized to build components such as Micro Electromechanical Systems (MEMS), also known as Microsystems Technology (MST), that provide various different functions in numerous different devices. For example, MEMS may be used in inkjet printers, pressure sensors, crash sensors in vehicles, accelerometers, gyros, inertial instruments, and so forth. An example of such a device is shown in FIG. 1. In this instance, two silicon-on-insulator (SOI) wafers 101 and 105 surround a center wafer 103 The device layers on each of these SOI wafers are etched to provide the appropriate function desired for the component. Two different ways of bonding semiconductor wafers are predominantly utilized: fusion bonding and anodic bonding.
Fusion bonding is performed by hydrolyzing the surfaces of the wafers, aligning the wafers relative to each other as necessary, and contacting the wafers together. The wafers are then brought to a fusion bonding temperature, typically between 300° C. and 800° C., and subsequently annealed at a higher temperature such as 800° C. to 1100° C. to increase the bond strength.
Anodic bonding is a process that joins together a silicon wafer and a sodium-containing glass substrate having similar coefficients of thermal expansion. Bonding is performed at a temperature between 200° C. and 500° C. while a voltage (500 to 1500 V DC) is applied across the substrates. The glass substrate is held at a negative potential, causing positive sodium ions to be mobile in the heated glass and to migrate away from the silicon-glass interface toward the cathode, leaving behind negative fixed charges. Bonding is complete when the ion current vanishes, indicating that a layer depleted of mobile sodium ions have been produced and non-bridging oxygen atoms have attached to silicon atoms to form silicon dioxide bonds. Anodic bonding is also known as electrostatic bonding.
Because the co-efficient of thermal expansion between glass and silicon in anodic bonding is not identical, thermal stresses may result in the manufacture of a MEMS device that experiences impaired performance over temperature. Depending on cleanliness and contact conditions, the fusion bonding process may cause trapped contaminants and gas pockets that result in poorly bonded areas, i.e., the bond strengths vary across the wafer. Although fusion bonding is often utilized to manufacture silicon MEMS, such devices may not have consistent bond strengths through the wafer due to the discontinuous bond areas caused by etching of the desired devices. Therefore, an improved method of wafer bonding for a MEMS devices and SOI wafer production is desired.
Accordingly, there is a need for a method of wafer bonding that provides a strong bond with minimal thermal stresses.